4.2.1 Soldier piles
Types of steel for soldier piles include the rail pile，the steel H⁃pile（or W section）and the steel I⁃pile（or S section）.The rail pile and the steel H⁃pile are more commonly used than the steel I⁃pile.It is optional to place laggings between soldier piles.Whether to place them or not depends on the in situ soil properties and strength characteristics.As Figure 4.1 illustrates，the construction procedure for soldier piles can be described as follows：
Figure 4.1 Soldier piles
①Strike soldier piles into soil.In non⁃urban areas，it will be all right to strike them into soil directly.In urban areas，however，static vibrating installation would be a better way to have soldier piles penetrated into the soil.If encountering a hard soil layer，pre⁃bore the soil.
②Place laggings as excavation proceeds.Then backfill the voids between soldier piles and laggings.
③Install horizontal struts in proper places during the excavation process.
④Excavation completed，begin constructing the inner walls of the basement.Then remove struts level by level and construct floor slabs.
⑤Complete the base.
⑥Pull out the piles.
The merits of soldier piles include：
a.Easier and faster construction with lower cost.
b.Piles can be easily pulled out.
c.Less ground disturbance is caused when pulling out the piles，compared to pulling out sheet piles.
d.The pile tip can be strengthened with special materials used in gravel soils.
e.Soldier piles are reusable.
The drawbacks of soldier piles include：
a.Sealing is difficult.In sandy soils with high groundwater level，some dewatering measures may be necessary.
Figure 4.2 Photo of the soldier pile method（Ou，2006）
b.Installing soldier piles by striking will cause much noise and vibration.The latter will render sandy soils below the foundation denser in such a way that uneven settlement of the adjacent buildings may occur.
c.Backfilling is necessary if soldier piles are driven using pre⁃boring.Deficiency in backfilling will cause bad effects in the vicinity.
d.The voids between retaining walls and surrounding soil need filling.
Figure 4.2 shows construction with the soldier pile method.